The present invention relates to a method of improving an optical disk device, for a ROM disk, a writing once disk, or an erasable disk for high-density recording, in which a focusing offset for the best reproduction signal is different from a focusing offset for the best tracking error signal and, more particularly, to an astigmatic difference correcting method for an optical head used in an optical disk device for high-density recording or the like and an apparatus therefor.
An optical system for a conventional optical head has no provision for correcting an astigmatic difference of a semiconductor laser. In the optical system, the focusing offset which will achieve the best reproduction signal (RF signal) is generally different from that which will achieve the best tracking error signal.
FIGS. 15 and 16 respectively show the best RF signal and the best tracking error signal when the levels of a digital reproduction (RF) signal which forms an eye pattern and a tracking error (TE) signal are changed with a change in focusing offset. As is apparent from FIGS. 15 and 16, when a focused beam has an astigmatic difference due to a change in focusing offset (the degree to which the beam is out of focus), a modulation transfer function (to be referred to as an MTF hereinafter) in the tangential direction of an optical disk increases. That is, it is understood that the best RF signals and best tracking error signals of beams focused in the tangential and radial directions of the disk are different conditions and these situations depend on focusing offsets. More specifically, when an astigmatic difference occurs due to a focusing position of a focused beam, the beam becomes an elliptic beam, and the lengths of the major and minor axes are different from each other. For this reason, MTFs are different in the tangential and radial directions of the disk which are perpendicular to each other. An RF signal is maximum when the beam is focused in the tangential direction of the disk, and the best tracking error signal can be obtained when the beam is focused in the radial direction of the disk. In this case, when the maximum and minimum outputs of a reproduction signal from the optical disk are represented by A and B, respectively, an MTF is expressed by MTF=(A-B)/(A+B).
It is important that a tracking servo operation is stabilized and a good RF signal is obtained in an optical head designed to have a high recording density and a high transfer rate.
In an optical head, the best focusing offset position required to maximize a tracking error signal is different from the focusing offset position required to maximize an RF signal. Therefore, although a tracking servo operation is stabilized when the tracking error signal is best, a good signal cannot be reproduced when focusing position is set for optimum tracking in MTF. In contrast to this, when a focusing offset is set to obtain the best RF signal, a tracking operation is disadvantageously performed. A serious problem in which a focusing offset for the best tracking error signal is different from a focusing offset for the best RF signal is supposed to be posed by the astigmatic difference of a focused beam.